Electrode support for arc lamps

ABSTRACT

An electrode support structure is disclosed for use in a short arc lamp. The structure includes a ring having a plurality of flaps formed therein. Struts are attached to the electrode and to the flaps. These struts may be straight and may be abutted to the electrode. The flaps can bend to minimize the stresses created by the contraction and expansion of the struts relative to the ring during the temperature cycling of assembly and operation.

United States Patent 1 Lavering 1 Feb. 6, 1973 [54] ELECTRODE SUPPORTFOR ARC LAMPS [75] Inventor: Gordon R. Lavering, Belmont, Calif. [73]Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: May 14, I971 [21] Appl. No; 143,377

52 US. Cl ..313 204, 313/285 [Sl] Int. Cl ..H0lj 1/96, H01] 19/50 [58]Field of Search ..3l3/l 13, 285, 205, 204, 226

[56] References Cited UNITED STATES PATENTS l2/l970 Peacher ..3l3/ll32/1970 Richter ..3 l3/ll3X 2/1970 Richter ..'.....313/113x 3/1970Richter ..313/113x Prima ry ExaminerAlfred L. Brody Att0meyStanley Z.Cole 57 ABSTRACT An electrode support structure is disclosed for use ina short arc lamp. The structure includes a ring having a plurality offlaps formed therein. Struts are attached to the electrode and to theflapsl These struts may be straight and may be abutted to the electrode.The

flaps can bend to minimize the stresses created by thecontraction andexpansion of the struts relative to the ring during the temperaturecycling of assembly and operation.

11 Claims, 5 Drawing Figures ELECTRODE SUPPORT FOR ARC LAMPS BACKGROUNDOF THE INVENTION This invention relates to gaseous discharge devices andin particular to an improved support for an electrode in a sealed beamshort are lamp. The invention herein described was made in the course ofor under a contract with the Department of Defense.

In one common short are lamp design, one electrode is supported adjacentto the lamps window. Such a lamp is described in detail in copendingapplication Ser. No. 143,166 filed May 13, 1971 and assigned to the sameassignee as the present application. In order to minimize the amount oflight intercepted by the structure supporting this electrode, thinelectrode supports are used. Prior art supports are thoroughly describedin U.S. Pat. No. 3,495,118 issued Feb. 10, 1970. Two basic types areusedstraight or arcuate. Both types are attached to some sort of ringwhich is part of the lamp envelope or is attached thereto. As the strutand ring are heated during assembly, the radius of the ring increasesmore than the length of the strut for typical materials used. Forexample, with molybdenum struts and a stainless steel ring, thedifferential expansion is about 1- percent. As a result some means mustbe provided to relieve the stress caused by this differential expansion.During operation the strut is heated more than the ring so thatadditional relative expansion or contraction will usually occur.

The prior art straight struts are attached tangentially to theelectrodes. As the ring expands relative to the struts during theheating of assembly, the struts slide tangentially across the surface ofthe electrode or draw partially out of the braze slot in the ring untilthe bond is formed at a high temperature. Thenas the structure coolsdown, a compressive force is applied endwise on the struts. A comparablestrain must occur in the assembly. The ring is much stronger than thestruts, so the ring deforms very little. The compressive stress on thestrut is not directly supported at the electrode end. Instead, a bendingmoment is set up because the strut is attached to the outside of theelectrode but the resultant of' forces passes through the center of theelectrode. Because of the strong ring, the strut must buckle (which itcan do easily if the strut is'arcuate) or the electrode rotates, bendingthe strut locally at the edge of the brazement between strut andelectrode. Bending proceeds until the compressive stress in the strut isequally opposed by the bending moment in the strut. However, stress isnot relieved. Instead stress builds up and is limited by the weakestpart of the system, namely, the bent part of the strut, which is weakestbecause it is hot and very ductile when the lamp heats up. Additionally,the stresses in the bonds between members will increase. Duringoperation of the lamp, the electrode again rotates and the struts bendto similarly transform the stresses in the members resulting from therelative contraction and expansion of the struts. The stresses in thebonds also remain, but at some changed value.

Arcuate struts can be abutted to the electrode and are under stress whenplaced in the brazing fixture. As the ring expands relative to thestruts during the heating ofassembly, the struts unbend to relieve thestresses described above. On cooling down the relative expansion of thestruts causes them to return to their approxi mate original shape inwhich they are again under some stress, as are the bonds. Duringoperation similar bending and unbending occur to minimize the stressesin the members caused by temperature cycling, although stresses in thebonds remain. The arcuate struts can also be tangentially attached tothe electrode with structural stresses relieved by a combination ofbending and rotation as described above.

In addition to the inadequate stress relief provided by the prior artelectrode supports, they are difficult to as- SUMMARY OF THE INVENTIONThe present invention is an improved support structure for an electrodein a short are lamp. The support structure comprises thin strutsattached to the electrode, a ring forming part of or attached to thelamp envelope, and flap portions in the ring, with each strut attachedto a separate flap portion. In a preferred embodiment, the flap portionsare formed from the ring in a rectangular shape, the struts are abuttedto the surface of the electrode and the free end of the flap portionsare attached to the struts at one end thereof. The struts can be abuttedto the flaps or the flaps can be abutted to the struts.

The primary object of this invention is to provide means for minimizingthe residual stress in a straight strut in an electrode supportstructure which allows the electrode to remain stationary during thermalcycling, and which results in deformation in a desirable location. Thisis accomplished by attaching the outer ends of the struts to flapportions in a ring. As the struts contract and expand relative to thering during the temperature cycling of assembly and operation, the flapsbend as required to minimize stress in the struts. The flaps can bedesigned to support whatever bending stress is desired.

BRIEF DESCRIPTION OF THE DRAWINGS form of attachment of the strut to theflap portion of.

the ring.

FIG. 5 is a cross-sectional view, partially schematic,

of a short are lamp employing the embodiment of FIG. 1.

DESCRIPTION'OF THE PREFERRED EMBODIMENTS FIGS. 1-3 illustrate apreferred embodiment of the electrode support structure of the presentinvention. Electrode l is supported by struts 3, usually three in numberto provide stability. Struts 3 are thin straight members. They are thinto minimize the interference with the light being projected past them.They' are straight because this is the most convenient and economicalform in which to produce them. As men-. tioned earlier, ease of assemblymakes it desirable to abut struts 3 to the electrode 1 rather thanattach them tangentially as in the prior art. This abutment can be donedirectly but use of a slotted sleeve 2 attached to electrode 1 makes iteasy to position struts 3 during assembly.

The other endsof struts 3 are to be attached to a ring 7. This ring 7either forms part of the lamp envelope or is attached to it. In theprior art the struts were typically abutted to a nonflexible ring. Inthis embodiment of the present invention a flap 6 is cut in the lowerportion 5 of ring 7. As detailed in FIG. 3, the open end of flap 6 isabutted to the side of strut 3 adjacent to the end of strut Ring 7 alsohas an upper portion 4. This upper por- 7 tion provides surfaces forattachment to other portions of the lamp, depending on the lamp design.It is thick enough to provide stability during temperature cycling. Thelower portion 5 of ring 7 is thin enough to permit the flaps 6 cuttherein to bend during temperature cycling, as described more fullybelow. The relative dimensions of the ring portions can vary dependingon lamp design including such factors as strut dimensions, operatingpressure of the lamp and which members are attached to the ring and.where they are attached. As an example, in one ring an inner diameter oftwo inches the thickness of lower portion 5 was about 0.05 inch, theheight of lower portion 5 was about 0.21 inch, the thickness of upperportion 4 (along a radius) was about gitudinal stress in the struts,caused by this relative contraction, is balanced by the stress set up bythe bending outward of flaps 6. Therefore at temperatures less thanbrazing temperature, flaps 6 and struts 3, as well as the bonds betweenthe various members, are under stress.

The flap is so dimensioned that the bending moment will not permitbuildup of the compressive stress in the strut to the point wherebuckling will occur. It is the object of the invention that the flap bedesigned to prevent the critical buckling stress in the strut from beingreached. Maximum fiber stresses in the flap may reach the yield pointsafely. If they do the flap will deform by plastic deformation inaddition to elastic deformation. In fact in a preferred embodiment theyield strength of the stainless steel ring is exceeded 10- cally in thebent region, resulting in a small amountof permanent deformation. Thefact that the strut is permitted to yield and that space is provided forit to yield means that there is a maximum compressive stress on thestrut that can be balanced by the stress in the bending fla'p. Thismaximum compressive stressis limited by the maximum bending moment thatcan be achieved in the flap, which is in turn, limited by the yieldstrength of the flap material. During operation of the lamp, struts 3are hottest adjacent to electrode 1. Relative expansion or contractionis difficult to predict because the temperature gradient is not knownbut excessive buildup of stress in struts 3 is prevented as flaps 6 bendas required.

As shown in FIG. 4, the end of struts 3 can also be abutted to theinside of flaps 6. Because ring 7 will expand relative to struts 3during the heating of assembly,

' when the structure is first placed in the brazing fixture,

0.14 inch, the height of upper portion 4 at its outer diameter was about0.12 inch with the height at its inner diameter about 0.06 inch, thewidth of flap 6 was about 0.15 inch, and the length of flap 6 was about0.5 inch.

In assembling the electrode support structure, the structure is heatedto a temperature sufficient to form the bonds between struts 3 andelectrode 1 and between struts 3 and flaps 6. Typically this is done bybrazing. As the structure is heated, in addition to other directionselectrode 1 expands radially, struts3 expand longitudinally and ring 7expands radially. However, the radial expansion of ring 7 is more thanthe sum of the longitudinal expansion of struts 3 and the radial expansion of electrode 1 for typical materials-brazingly compatible withthe tungsten electrode required by the extreme heat generated at theelectrode tip, such as molybdenum struts and a stainless steel ring. Asa result flaps 6,move outward relative to the ends of struts 3, untilthebondsbetw'een flaps 6 and struts 3 form. Struts 3 must besufficiently longer than the distance from the electrode 1 to the'outersurface of flap 6 to accommodate this relative expansion of ring 7: Asthe strucflaps 6 are forced outward 'by the extra length of struts 3.Then as the structure is heated flaps 6 tend to return to their originalposition but on cooling down they are The embodiment in FIG. 3 allowsfor loose tolerancein the lengths, widths and thicknesses of struts 3but some means is required during brazing to insure that the end offlaps 6 are in contact with struts 3. On the other hand, the embodimentin FIG. 4 requires close tolerance in the lengths of struts 3 butassembly is easi- FIG. 5 illustrates a short are lamp showing theelectrode support structure of FIG. 1 in place. Details of constructionof the lamp are not shown, aside from those in the electrode supportstructure. Such a lamp is described in more detail in copending U.S.Patent application Ser. No. 143,166 filed May 13, 1971, and assigned tothe same assignee as the present invention. The lamp has acylindricalmember 52 which, together with window 54 and base 53, forms a sealedenvelope filled with gas under high pressure. A reflector 55 is attachedto ring 7. A second electrode 51 is supported by appropriate means frombase 53.

The shape of ring 7 can vary as required by the particular design of thelamp into which the electrode support structure is to be put. Someportion of the ring must be thick enough to provide structural stabilityand must have the proper surface for attachment to the rest of the lamp.In the embodiment just described, these functions were served by upperportion 4. Some portion of ring 7 must be thin enough to accommodate theflap portions 6 and to allow space for the required bending. In theembodiment just described, this func- .long, the flap can vibrate whenthe lamp is jarred even in normal use, thus causing the electrode tomove relative to the reflector focal point, causing beam swing. A changein the arc gap can cause the arc to be extinguished. A length to widthratio for the flap of 3 to 4 has been found effective and convenient.The position of the flap relative to an edge of the ring is also amatter of convenience, with positions possible other than ones whichhave a-coterminous edge.

What is claimed is:

1. An electrode support structure in a short are lamp comprising anelectrode, a ring coaxial with and spaced from said electrode, said ringhaving a plurality of flap portions thereon, and a plurality ofelectrode support struts, each of said struts being attached to adifferent one of said flap portions and to said electrode whereby saidflap portions can more relative to the remaining portions of said ringto accommodate relative movement between said remaining portions of thering and said struts.

2. An electrode support structure as claimed in claim 3 wherein saidstruts are of molybdenum and said ring is of stainless steel.

5. An electrode support structure as claimed in claim 1 wherein saidflap portions are rectangular shaped.

6. An electrode support structure as claimed in claim 5 wherein saidrectangular-shaped flap portion is coterminous with said ring at onesidealong a line substantially parallel to the axis of said ring.

7. An electrode support structure as claimed in claim 5 wherein theratio of length to'width of said flap portion is in the range 3.0 to4.0. v

8. An electrode support structure as claimed in claim 1 wherein saidflap portions are abutted to the sides of said struts.

9. An electrode support structure as claimed in claim 1 wherein saidflap portions each have a groove formed therein and said struts areabutted to said flap portions in said grooves.

10. An arc lamp comprising:

a sealed envelope including an optical window at one end thereof, twoelectrodes supported within said envelope and spaced apart to define ashort are gap therebetween, with the means of support of at least one ofsaid electrodes comprising a ring coaxial with and spaced from saidelectrode, said ring having a plurality of flap portions formed therein,and a plurality of electrode support struts, each of said struts beingattached to a different one of said flap portions and to said electrode,

an ionizable gas at least filling said are gap, and a reflectorsupported within said envelope whereby said flap portions can morerelative to the remaining'portions of said ring to accommodate relativemovement between said remaining portions of the ring and said struts.

11. An electrode support structure as claimed in claim 1 wherein each ofsaid flap portions comprises an elongated spring member having one endthereof attached to said remaining portion and having the longitudinalaxis thereof oriented substantially circumferentially of said ring.

1. An electrode support structure in a short arc lamp comprising anelectrode, a ring coaxial with and spaced from said electrode, said ringhaving a plurality of flap portions thereon, and a plurality ofelectrode support struts, each of said struts being attached to adifferent one of said flap portions and to said electrode whereby saidflap portions can more relative to the remaining portions of said ringto accommodate relative movement between said remaining poRtions of thering and said struts.
 1. An electrode support structure in a short arclamp comprising an electrode, a ring coaxial with and spaced from saidelectrode, said ring having a plurality of flap portions thereon, and aplurality of electrode support struts, each of said struts beingattached to a different one of said flap portions and to said electrodewhereby said flap portions can more relative to the remaining portionsof said ring to accommodate relative movement between said remainingpoRtions of the ring and said struts.
 2. An electrode support structureas claimed in claim 1 wherein said struts are straight and one end ofeach of said struts is abutted to said electrode.
 3. An electrodesupport structure as claimed in claim 2 wherein a slotted sleeve isattached to said electrode and said struts are abutted to said electrodein the slots of said sleeve.
 4. An electrode support structure asclaimed in claim 3 wherein said struts are of molybdenum and said ringis of stainless steel.
 5. An electrode support structure as claimed inclaim 1 wherein said flap portions are rectangular shaped.
 6. Anelectrode support structure as claimed in claim 5 wherein saidrectangular-shaped flap portion is coterminous with said ring at oneside along a line substantially parallel to the axis of said ring.
 7. Anelectrode support structure as claimed in claim 5 wherein the ratio oflength to width of said flap portion is in the range 3.0 to 4.0.
 8. Anelectrode support structure as claimed in claim 1 wherein said flapportions are abutted to the sides of said struts.
 9. An electrodesupport structure as claimed in claim 1 wherein said flap portions eachhave a groove formed therein and said struts are abutted to said flapportions in said grooves.
 10. An arc lamp comprising: a sealed envelopeincluding an optical window at one end thereof, two electrodes supportedwithin said envelope and spaced apart to define a short arc gaptherebetween, with the means of support of at least one of saidelectrodes comprising a ring coaxial with and spaced from saidelectrode, said ring having a plurality of flap portions formed therein,and a plurality of electrode support struts, each of said struts beingattached to a different one of said flap portions and to said electrode,an ionizable gas at least filling said arc gap, and a reflectorsupported within said envelope whereby said flap portions can morerelative to the remaining portions of said ring to accommodate relativemovement between said remaining portions of the ring and said struts.